FRIDAY, Feb. 12, 2016 (HealthDay News) -- Women with asthma may take longer to get pregnant and have a lower pregnancy rate than those without the lung disease, new research suggests.

The study included 245 women, aged 23 to 45, who had unexplained fertility problems and were undergoing fertility treatment. Ninety-six of the women had been diagnosed with asthma.

The women were followed until they had a successful pregnancy, stopped treatment or the study ended. The median time for women without asthma to get pregnant was about 32 months compared to more than 55 months for those with asthma. Median means half took more time to conceive; half, less.

About 60 percent of women without asthma got pregnant, compared with just under 40 percent of those with asthma, the findings showed. The gap between the two groups increased with age, according to the study published Feb. 12 in the European Respiratory Journal.

The trial finding adds new weight to evidence suggesting a link between asthma and fertility, lead author Dr. Elisabeth Juul Gade said in a journal news release. Gade is with the department of respiratory medicine at Bispebjerg University Hospital in Copenhagen, Denmark.

"We have seen here that asthma seems to have a negative influence on fertility as it increases time to pregnancy and even more so with age," she said. "We do not yet know the causal relationship; it may be complex with different types of asthma, psychological well-being, asthma medication and hormones all playing a role."

Gade said doctors should encourage women with asthma to become pregnant at an earlier age and step up their asthma treatment before conceiving.

"Patient education is also of paramount importance as adherence to treatment may be enhanced if patients are informed of this link," Gade said in the news release.

While the study found an association between asthma and difficulty conceiving, it did not prove cause-and-effect.